Definition
The unified atomic mass unit , u, is defined as 1/12th of the rest mass of a neutral atom of 12C in its nuclear and electronic ground state. Avogadro’s constant, N A, is the number of atoms in a mole of substance, where mole, denoted mol, is defined as the amount of substance that contains as many elementary entities as there are atoms in 0.012 kg of carbon-12.
Unified Atomic Mass Unit
The nineteenth-century chemists had two different standards for atomic weight values: hydrogen = 1, as chosen and quoted by John Dalton (1805), and oxygen = 16, as suggested by Brauner (1889) and the German Atomic Weights’ Committee (Ostwald, 1903) because oxygen was a primary measurement standard. At the start of the twentieth century, chemists (Landolt et al., 1900) voted for oxygen =16. The physicist, Francis Aston, who was using a mass spectrometer to determine mass ratios of isotopes (nuclides), used the stable isotope 16O = 16 as a standard. With the discovery (Giaugue and Johnson, 1929a...
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References
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Holden, N.E. (2016). Unified Atomic Mass Unit, Avogadro Constant, and Mole. In: White, W. (eds) Encyclopedia of Geochemistry. Encyclopedia of Earth Sciences Series. Springer, Cham. https://doi.org/10.1007/978-3-319-39193-9_233-1
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DOI: https://doi.org/10.1007/978-3-319-39193-9_233-1
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